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No. 9174 



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Bureau of Mines Information Circular/1988 




Review of Membrane Technology 
for Methane Recovery From Mining 
Operations 



By F. Garcia and J. Cervik 




UNITED STATES DEPARTMENT OF THE INTERIOR 



Information Circular 9174 



Review of Membrane Technology 
for Methane Recovery From Mining 
Operations 



By F. Garcia and J. Cervik 



UNITED STATES DEPARTMENT OF THE INTERIOR 
Donald Paul Hodel, Secretary 

BUREAU OF MINES 

David S. Brown, Acting Director 



MS 



Library of Congress Cataloging in Publication Data: 



Garcia, F. (Fred) 

Membrane technology and methane recovery from mining operations. 



(Information circular ; 9174) 

Bibliography: p. 6 

Supt. of Docs, no.: I 28.27: 9174. 

1. Coalbed methane. 2. Membranes (Technology). I. Cervik, Joseph. II. Title. 
III. Series: Information circular (United States. Bureau of Mines) ; 9174. 

-JFN295.U4- 622 s [622 '.334] 87-600370 [TN305] 



CONTENTS 

Page 

Abstract 1 

Introduction 2 

History of membrane technology 2 

Gas transport through membranes 3 

Possibilities for applying membrane technology to mining 5 

Conclusions 6 

References 6 

ILLUSTRATIONS 

1. Hollow-fiber membrane 3 

2. Spiral-wound membrane 3 

TABLE 

1. Flammability limits of natural gas-air mixtures 5 





UNIT OF MEASURE 


ABBREVIATIONS USED 


IN THIS REPORT 


cm 


centimeter 


lb/in 2 


pound (force) per 
square inch 


ft 


foot 










lb/in 2 (ga) 


pound (force) per 


ft 3 


cubic foot 




square inch, gauge 


gal 


gallon 


m 


meter 


hp 


horsepower 


m 3 


cubic meter 


in 


inch 


pet 


percent 


kPa 


kilopascal 


St 


short ton 


kW 


kilowatt 


vol pet 


volume percent 


lb 


pound 







REVIEW OF MEMBRANE TECHNOLOGY FOR METHANE 
RECOVERY FROM MINING OPERATIONS 

By F. Garcia 1 and J. Cervik 2 



ABSTRACT 

Recent advances in the commercial separation of gases using membranes 
have renewed interest in the possibility of applying this technology to 
the recovery of methane (CH4) from mining operations. This Bureau of 
Mines report briefly reviews the history of the development of membranes 
for gas separation, the theory of how they work, and their application 
to the separation of methane from air and associated problems. 

However, methane-air mixtures are difficult to separate with membranes 
because the pertinent gas couples, 02 - N2, O2-CH4, and N2-CH4, have poor 
separation characteristics, as indicated by their separation factors of 
about 3 or less. Even if these separation factors were substantially 
higher, there is doubt that methane could be recovered economically from 
the low concentrations in mine ventilation exhaust (2 vol pet or less). 
The exhaust pressures are not sufficient for adequate separation. The 
power cost of compressing these mixtures would far exceed the value of 
the methane recovered. 

New discoveries could make separation of gob hole methane-air mixtures 
practical. These mixtures have much higher concentrations of methane 
(from 30 to 100 vol pet); however, for safety reasons, treatment would 
be limited to gob gas with 60 vol pet CH4 or more. 



'Mining engineer. 
^Supervisory geophysicist. 
Pittsburgh Research Center, Bureau of Mines, Pittsburgh, PA. 



INTRODUCTION 



A Bureau of Mines study showed that in 
1985 coal mines in the United States 
emitted 304 million ft (8.6 million m ) 
of methane daily (1). This reflects an 
increase of about 48 million ft (1.4 
million m ) since 1980 (2). Factors con- 
tributing to this increase were the open- 
ing of new mines in deeper and gassier 
coal beds in the last ten years and these 
mines are now larger mines. 

Ventilation is the primary method of 
controlling methane in coal mines. At an 
active coal face, the methane must be 
diluted with air to 1 vol pet CH4 or less 



for safety reasons. In return airways 
and bleeders, the maximum allowable 
methane concentration is 2 vol pet. The 
methane in gas vented from coal mines 
cannot be recovered economically with 
present technology such as distillation, 
absorption, and adsorption (3_). 

Cryogenic distillation has always been 
the system of choice for large-scale sep- 
aration of gases as well as liquids (4^). 
However, advances in membrane technology 
in the past 6 yr have spurred interest in 
and wide industrial use of membranes for 
gas separation. 



HISTORY OF MEMBRANE TECHNOLOGY 



A simple approach to separating a gas 
mixture is to construct a barrier that 
permits molecules of one kind to pass 
through it while excluding others. Such 
a barrier in the form of a membrane was 
first reported in 1831 (5). Thirty-five 
years later, the mechanism of permeation 
through a membrane was discussed and dem- 
onstrated by using a rubber membrane to 
separate a gas mixture. 

Polymer membranes were introduced in 
the late 1940's. These were composite 
membranes that consisted of a very porous 
but inert substrate covered by. a polymer 
layer that separated components. Certain 
uses of these membranes are well estab- 
lished, and many improvements in the per- 
formance of these membranes have been 
achieved (6^). For example, large-scale 
water-desalination plants can process 
more than 600 million gal (2.3 million 
m ) of water daily; the dairy industry 
uses membrane technology to process whey 
proteins; and hemodialyses, a standard 
treatment for patients with kidney fail- 
ure, depends upon membrane technology. 

The possibility of using membranes for 
industrial gas separation became evident 
in the 1950 's with the development of new 
polymeric materials. Many industrial gas 
separation processes were examined to 
determine if the use of membranes could 

— _ 

-"Underlined numbers in parentheses re- 
fer to items in the list of references at 
the end of this report. 



be applied to them. These processes in- 
cluded separation of O2 from air, He from 
natural gas, H2 from coal-hydrogenation 
tail gas and refinery gas, NH4 from mix- 
tures containing N2 and H2, and CO2 from 
various gas mixtures. The use of mem- 
branes in these industrial processes was 
limited severely by low permeation rates 
through the membrane and the poor 
membrane durability under operational 
conditions. 

A major breakthrough in membrane tech- 
nology occurred in 1960 with the develop- 
ment of asymmetric membranes (]_)• These 
membranes are porous throughout , but have 
a thin, relatively dense skin near one 
surface, which generally accounts for a 
very small fraction (0.1 to 1 pet) of the 
total membrane thickness (4^). These mem- 
branes have proportionally higher perme- 
ation rates than the dense membranes of 
equivalent thickness because the effec- 
tive separating layer (the dense skin) is 
so thin. However, they contain pores 
that are substantially larger than gas 
molecules, and as a result, they make 
poor gas separators. 

In the mid-1970's, a new process de- 
veloped by Monsanto Co., St. Louis, MO, 
overcame the problem of surface porosity 
in asymmetric membranes by applying a 
high-permeability coating to the porous 
membrane. The coating plugged surface 
pores and also served to protect the 
substrate from damage due to abrasion 
and normal handling (]_)» Because of the 



Nonpermeative gas outlet 



Fiber bundle 
plug 



4- or 8-in 
diam by 10 ft 



Feed stream 

of 
mixed gases 




Hollow-fiber 
membrane 



ASME code carbon 
steel shell 



Not to scale 



Permeative gas outlet 

FIGURE 1 .—Hollow-fiber membrane. 



coating, the gas-separating layer can be 
made thin without concern regarding pore 
problems and consequently, gas flow rates 
through the substrate are 1,000 to 10,000 
times faster than through other types of 
membranes. 

The Monsanto membrane 4 can be manu- 
factured in flat sheets or as a hollow- 
fiber membrane (8). The two membrane 
types have different configurations. 
Hollow-fiber membranes are slender, spun 



Feed 



Permeative at 
low pressure 



Residual gas 



KEY 

1 Feed channel 

2 Membrane 




Feed channel 
Membrane 
-Permeative channel 



FIGURE 2.— Spiral-wound membrane. 

filaments several hundred microns in 
diameter. Typically, they are packaged 
in parallel in a 4- or 8-in-diam (10.2- 
or 20.3-cm) steel tube (module) into 
which the gas mixture is forced. The gas 
to be separated permeates from the out- 
side to the inside of the hollow fibers 
(or vice versa) and is collected at one 
end of the tube (fig. 1). To obtain a 
spiral-wound configuration, several flat 
sheets are separated by spacers to create 
a turbulent flow path for the feed gas, 
and then rolled up on a central tube and 
inserted into a steel shell (module) 
(fig. 2). Both hollow-fiber and spiral- 
wound modules are arranged into various 
interconnected banks to constitute a sep- 
aration system. 

Monsanto-type members are used commer- 
cially for the separation of H2 and He 
from gases such as N2, CO, and CH4 (4_). 
Potential applications for these mem- 
branes include H2 recovery from purge 
gases in ammonia synthesis and the recov- 
ery and recycling of CO 2 in enhanced oil 
recovery processes. 



GAS TRANSPORT THROUGH MEMBRANES 



Gas transport through a membrane is 
controlled by Fick's law of diffusion and 
Henry's law relating solubility of a gas 
in the polymeric membrane (4, 9). By 
Fick's law, diffusion through the mem- 
brane is 

^Reference tc specific products does 
not imply endorsement by the Bureau of 
Mines. 



Q = DaAACa ) 



(1) 



where Q = flow of component a through 
membrane , 

D a = diffusion coefticient for 
component a, 



AC a = concentration difference 
across the membrane, 

A = membrane surface area, 

and L = membrane thickness. 

Henry's law relates the concentration of 
gas a to the partial pressure of gas a in 
contact with the polymer: 

Ca - S a P a , (2) 

where S a = solubility constant 

and P a = partial pressure in contact 
with membrane. 

Substituting equation 2 into equation 1 
yields 

Q= KaAAPa, (3) 

Li 

where K a = S a D a = permeability 
coefficient. 

An equation of this form can be written 
for each component in the gas stream. 
Equation 3 shows that where the partial 
pressure differentials for two gases in 
a mixture are the same, the ratio of the 
flow rates of each gas through the mem- 
brane may be expressed as 



Qa 
Qb 



Ka 
Kb 



(4) 



where a = selectivity or separation 
factor. 

The separation factor indicates the abil- 
ity of the polymer to separate two gases 
in a given mixture. 

Equation 3 shows that high gas flows 
through a membrane can be obtained by in- 
creasing the permeation coefficient (K), 
or increasing the surface area of the 
membrane (A), or increasing gas pressure 
(P). The problems associated with the 
control of the physical and separation 
properties (K) of polymers can be almost 
as great as those associated with mak- 
ing working membranes (^0. Consequently, 
commercial membrane separation systems 
are based on available polymers developed 



for other applications. Increasing the 
surface area of a membrane increases the 
size and cost of the system and at some 
point, which is specific for each appli- 
cation, use of additional surface area 
makes the system uneconomical. Gas flow 
through a membrane requires a driving 
force; this force is represented in 
equation 3 by the partial pressure dif- 
ferential across the membrane (AP a ). Gas 
flow through the membrane can be in- 
creased by increasing gas pressure. How- 
ever, compression consumes energy; and 
the increased cost of pressure vessels, 
compressors , and energy associated with 
compression makes this approach unattrac- 
tive. In addition, higher operating 
pressures require a membrane of much 
greater strength. Thus, practical and 
economic problems limit the degree to 
which the permeation coefficient, mem- 
brane surface area, and gas pressure can 
be changed. 

Equation 3 shows that gas flow rates 
through a membrane are inversely propor- 
tional to the thickness (L) of the mem- 
brane. Thus, the development of thinner 
polymer membranes made possible the pro- 
duction of high-gas-flow systems that 
satisfy the demands of the commercial gas 
separating industry. For example, the 
overall thickness of the Monsanto mem- 
brane ranges from 1 by 10~ 3 to 10 by 10~ 3 
in (2.5 by 10~ 3 to 25 by 10" 3 cm). Its 
dense skin, which actually accomplishes 
the gas separation, is 0.004 by 10" 3 to 
0.04 by 10" 3 in (0.01 by 10" 3 to 0.1 by 
10" 3 cm) thick (4_). 

The selectivity or separation factor (a 
in equation 4) should be at least 20 and 
often must be more than 40 for the gases 
to be separated (4^). The separation fac- 
tors for various gas couples follow: 

H 2 (vapor)-CH 4 200-400 

H 2 -CH 4 40-55 

CO 2 -CH 4 20-30 

H 2 S-C 3 H 8 75-110 

He-CH 4 60-85 

O2-N2 4-5 



The O2-N2 separation factor is of an 
order of magnitude less than the separa- 
tion factors of the other gas couples 
because of the small differences between 



O2 and N2 molecular size and solubility 
(10) . This low separation factor makes 
O2 enrichment difficult to accomplish. 



POSSIBILITIES FOR APPLYING MEMBRANE TECHNOLOGY TO MINING 



Methane is exhausted from coal mines 
through the ventilation system and 
through surface gob holes. In some 
mines, long horizontal holes are drilled 
into the coalbed to drain methane. These 
holes are connected to an underground 
pipeline that transports the gas to the 
surface. Because the methane content of 
the drained gas is 90 vol pet or greater, 
it usually requires no remedial treatment 
and can be compressed and pumped into a 
commercial gas transmission pipeline. 
However, the gas exhausted through the 
ventilation system and produced from gob 
requires remedial treatment before it can 
be pumped into commercial pipelines. 

The main gas components of a CH4~air 
mixture are O2, N2, and CH4. The fol- 
lowing tabulation gives the permeation 
rates of these gases at 750 lb/in 2 (5,170 
kPa), in standard cubic feet per hour per 
square foot times 100 lb/in : 

CH 4 0.18 



N 2 . 



0.16 



2 0.59 

The permeation rates for N2 and CH 4 are 
practically the same, while the rate for 
O2 is almost three times greater than 
either CH4 or N2 (11). Consequently, the 
use of membranes to upgrade coal mine 
exhaust ventilation systems does not ap- 
pear to be feasible with existing mem- 
brane systems. Even if a membrane system 
existed that would separate the methane 
from the exhaust ventilation gas with 100 
vol pet efficiency, only 2 ft 3 (0.06 m 3 ) 
of methane at most would be obtained for 
every 100 ft 3 (2.8 m 3 ) of exhaust gas 
treated. Because membrane systems oper- 
ate at pressures of 2,000 lb/in 2 (13,790 
kPa) or more (4), the cost of energy re- 
quired to sufficiently compress the ex- 
haust gas far exceeds the value of the 



CH 4 recovered. For example, if the ex- 
haust gas is compressed to 2,000 lb/in , 
and 1 million ft 3 (0.028 million m 3 ) of 
gas were treated daily, a 700-hp (522 kW) 
compressor would be required. Electrical 
power to operate the compressor would 
cost about $525 daily, while the value of 
the recovered methane would be only about 
$130 based on a sale price of $6.50 per 
1,000 ft 3 ($6.50 per 28.8 m 3 ), a price 
that is higher than you can now get. 

Gob gas is generally mixed with mine 
air, and its composition varies from 
nearly 100 vol pet CH 4 when production 
first starts to 30 vol pet over a period 
of months. Because the separation fac- 
tors for the gas couples in the mixture 
are less than about 3, membrane tech- 
nology does not appear applicable to 
upgrade gob gas to a saleable product. 
There are safety factors to consider when 
compressing gob gas. Methane-air mix- 
tures are explosive in the range from 5 
to 15 vol pet CH4 at atmospheric condi- 
tions. Methane is almost always the 
major constituent of natural gas. Conse- 
quently, methane and natural gas have 
similar limits of flammability (table 1). 
At pressures of 2,000 lb/in the lower 
limit of flammability decreases to 3.60 
vol pet and the upper limit increases 
to 59.0 vol pet. Thus, if appropriate 

TABLE 1. - Flammability limits 
of natural gas-air mixtures 



Pressure, lb/in 2 (ga) 

(atmospheric) 

500 

1,000 

2,000 

3,000 

e Estimated. 



Source: Jones (12, p. 7). 



Limits, vol pet 
natural gas 




14.20 
44.2 
52.9 
59.0 
e 60.0 



60142 138 



membrane technology were available, only 
gob gas that contained more than 60 
vol pet CH4 could be treated. The re- 
mainder would have to be vented. 

The gases N 2 , 2 , and CH 4 rend to have 
low permeabilities in most polymers and 
are therefore difficult to separate. New 



discoveries will be needed to make such 
separations practical with membranes. 
Many new developments in both membranes 
and the process design of new applica- 
tions are expected in the next several 
years (4). 



CONCLUSIONS 



The present state of membrane technol- 
ogy precludes the use of membranes for 
separation of methane from exhaust venti- 
lation or gob gas. 

Because methane concentration in ex- 
haust ventilation is 2 vol pet or less, 
it is doubtful that this methane could be 
recovered economically even if membrane 
technology were available. The power 
cost of gas compression far exceeds the 



value of the methane that could be recov- 
ered. For gob gas, treatment would be 
limited to methane concentrations of 60 
vol pet or more because of the explo- 
sive nature of compressed methane-air 
mixtures. 

Membrane technology for gas separation 
is still developing. New membranes yet 
to be developed could make upgrading of 
gob gas feasible. 



REFERENCES 



1. Grau, R. H. An Overview of Methane 
Liberations From U.S. Coal Mines in the 
Last 15 Years. Paper in Proceedings 
of 3rd U.S. Mine Ventilation Symposium. 
Soc. Min. Eng. AIME, 1987, pp. 251-255. 

2. Irani, M. C, J. H. Jansky, P. W. 
Jeran, and G. L. Hassett. Methane Emis- 
sion From U.S. Coal Mines in 1975, A Sur- 
vey. BuMines IC 8733, 1977, 55 pp. 

3. Skow, M. L. , A. G. Kim, and 
M. Deul. Creating a Safer Environment in 
U.S. Coal Mines. The Bureau of Mines 
Methane Control Program, 1964-79. A Bu- 
reau of Mines Impact Report. BuMines 
Spec. Publ. , 1981, 50 pp. 

4. Henis, J. M. S., and M. K. Tripodi. 
The Developing Technology of Gas Separat- 
ing Membrane. Science, v. 220, No. 4592, 
1983, pp. 11-17. 

5. Lacey, R. , and S. Loeb (ed.). 
Industrial Processing With Membranes. 
Wiley, 1978, pp. 279-339. 

6. Fox, J. L. Membrane Development 
Slowed by Weak Economy. Chem. & Eng. 
News, v. 62, No. 52, Nov. 8, 1982, pp. 7- 
12. 



7. Rosenzweig, M. D. Unique Membrane 
System Spurs Gas Separation. Chem. Eng. 
(N.Y.), v. 88, No. 24, Nov. 1981, pp. 62- 
66. 

8. Parkinson, G. , S. Ushio, and 
R. Lewald. Membranes Widen Roles in Gas 
Separations. Chem. Eng. (N.Y.), v. 91, 
No. 8, Apr. 16, 1984, pp. 14-19. 

9. Maclean, D. L. , D. J. Stookey, and 
T. R. Metzger. Fundamentals of Gas Per- 
meation. Hydrocarbon Process., v. 62, 
No. 8, 1983, pp. 47-51. 

10. Schell, W. J., and C. D. Houston. 
Membrane Gas Separations for Chemical 
Processes and Energy Applications. ACS, 
1983, pp. 125-143. 

11. Schell, W. J. Membrane Use/Tech- 
nology Growing. Hydrocarbon Process., v. 
62, No. 8, 1983, 43 pp. 

12. Jones, G. W. , R. E. Kennedy, and 
I. Spolan. Effect of High Pressures on 
the Flammability of Natural Gas-Air- 
Nitrogen Mixtures. BuMines RI 4557, 
1949, 16 pp. 



U.S. GOVERNMENT PRINTING OFFICE: 1988 - 547-000/80.022 



INT.-BU.0F MINES, PGH., PA. 28655 



J.S. Department of the Interior 
Sureau of Mines— Prod, and Distr. 
Cochrane Mill Road 
P.O. Box 18070 
Pittsburgh. Pa. 15236 



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